In the regular FAQ it talks about Pixel anomalies.
To add to that, some blue and white pixels sometimes will not be completely dead. Sometimes if you use the cloth that came with your Flat Panel iMac, or another anti-static cloth, and gently rub the screen, a day later the dead pixel will become invisible.
Regarding "rubbing the screen" to get dead pixels to start working.
I just started the AppleCare service training course.
In the section regarding LCD displays, specifcally regarding "troubleshooting"
"Care of LCDs
Some precautions are necessary when using LCD displays.
Since glass is a main component of these displays, and since they are designed to minimize weight, it is easy to crack or break an LCD display panel if one is not careful.
Don't Rub the Screen
Some sources maintain that defective LCD pixels can be restored by rubbing the screen around the defective pixel. Given the great complexity of LCD displays, there are very great risks that such a procedure will simply make more pixels defective.
This procedure does not work and is very likely to create further problems. Don't rub LCD screens. If you have to clean an LCD screen surface, be sure to do so with a gentle pressure."
having read this, I would suggest that this advice be followed.
There is more info on dead pixels on apples website.
just some thoughts
When I say rub gently, I mean literally massage it. Don't press it hard at all. It just takes a very gentle touch. And then clean the area with iKlear